Single-phase compression and expansion radial (centrifugal) and mixed flow impellers are generally covered with an outer shell (cover or cap or fairing) so as to limit leak rates and recycle rates between the upper face and the lower face of the vanes and consequently to increase the impeller efficiency. These shells are generally provided, at one end thereof, with a seal (labyrinth seal for example) so as to limit leaks between the inlet and the outlet of the impeller as a result of the pressure gradient (positive in compression and negative in expansion) that appears during energy transformation.
Patent FR-2,697,870 describes the cover of vanes of compression helical axial flow impellers with a fairing itself covered on the total outer surface thereof by a seal system. The fairing has two purposes: first, it reduces the space between the rotor and the stator, considering the vane height reductions from the first to the last stage (volume flow rate reduction), second, it reduces leaks in the vicinity of each impeller while preventing friction losses by using a suitable seal system, for example grooves arranged in the direction of rotation.